Ionospheric Effects on Global Positioning System in Equatorial Africa: Impacts and Implications on Navigation.
A Thesis Submitted to the School of Postgraduate Studies, University of Lagos.
This thesis presents the results of study of equatorial ionospheric effects on Global Positioning System (GPS), impacts and implications to navigation in Africa. Despite some studies on scintillations and ionospheric total electron content elsewhere, significant work has not been carried out in the equatorial region of the African continent. Data used in this work were obtained from thirteen GPS stations, spanning the period 2000 to 2011. The data were analysed using various statistical tools. The results presented herein are, most likely, the first on the response of the equatorial vertical total electron content (VTEC) in the African sector during geomagnetic storm periods. A superposed epoch analysis of the storms was performed by defining the start time of the epoch as the storm-onset time. The results obtained showed that positive storm effects were more prevalent than the negative storm effects, and generally last longer, irrespective of storm onset-times. This work also dealt with the measurements of amplitude and phase scintillations. The results showed that the presence of large scale depletions of total electron content (TEC) or plasma bubbles were noted during the evening hours and that TEC depletion corresponded to increase rate of change of TEC (ROT). This confirmed that plasma bubbles are associated with large scale irregularities. It was also established that enhanced amplitude scintillation (S4) corresponded quite well with TEC depletions and increased ROT. The diurnal and seasonal percentage occurrence for different levels of scintillation activity had peaks in the equinox months (March, April, September, October and November) around 2000 - 2300 Local Time (LT). Diurnal, seasonal and latitudinal variations of VTEC over the equatorial region of the African continent and a comparison with International Reference Ionosphere 2007 (IRI 2007) derived TEC (IRI-TEC), using all three options, namely; NeQuick, IRI01-corr and IRI-2001, were also presented in this work. VTEC values were grouped into four seasons namely; March Equinox (February, March, April), June Solstice (May, June, July), September Equinox (August, September, October), and December Solstice (November, December, January). VTEC generally increased from 0600 h LT and reached its maximum value at approximately 1500 h to 1700 h LT during all seasons and at all locations. It was observed that the NeQuick option produced the best results during low solar activity (LSA) and medium solar activity (MSA) periods, while IRI01-cor produced the best results during high solar activity (HSA) over the equatorial region of Africa. The daily values of the 10.7 cm radio flux (F10.7) were used to represent solar Extreme Ultraviolet (EUV) variability in order to study the dependence of VTEC on solar activity variation. All analysed linear and quadratic fits demonstrated positive VTEC-F10.7 correlation, with all fits at 0000 h and 1400h LT being significant with a confidence level of 95% when both linear and quadratic models were used. All the fits at 0600 h LT were insignificant with a confidence level of 95%. The results of this study can be used to improve the IRI prediction of TEC around the equatorial region of the African sector. The results also shows that geomagnetic storms may affect GPS positional accuracy, navigational accuracy and the GPS tracking performance at the equatorial region of the African sector.